Within the U.S. Environmental Protection Agency (EPA), the
Office of Air Quality Planning and Standards (OAQPS) has the
responsibility for establishing and revising National Ambient Air
Quality Standards (NAAQS). In assessing alternative NAAQS proposed
for a particular pollutant, OAQPS evaluates the risks to human
health of air quality meeting each of the standards under
consideration. This determination of risk requires accurate
estimates of the number of persons exposed at various pollutant
concentrations for specified periods of time. The estimates may be
specific to an urbanized area such as Los Angeles or apply to the
entire nation.

Several researchers have recommended that such estimates be
obtained by simulating the movements of people through zones of
varying air quality so as to approximate the actual exposure
patterns of people living within a defined study area.
1,2,3 OAQPS has implemented this approach through an
evolving methodology referred to as the NAAQS Exposure Model or
NEM. From 1979 to 1988 OAQPS developed and applied
pollutant-specific versions of NEM to ozone,4 total
suspended particulate matter,5 and carbon
monoxide.6 These versions of NEM are referred to as
"deterministic" versions in that no attempt was made to model
random processes within the exposure simulation. An overview of the
evolution of the deterministic version of NEM is provided by Paul
et al.7